Heat pipe with composite capillary wick structure

ABSTRACT

A heat pipe includes a metal casing ( 100 ) and a composite capillary wick received in the casing. The casing has an evaporating section ( 400 ), a condensing section ( 600 ) and a central section ( 500 ) between the evaporating section and the condensing section. A first type of capillary wick ( 250 ) is provided in the central section of casing and a second type of capillary wick ( 240 ) is disposed at the evaporating section of the casing. The capillary pore size of the first type of capillary wick is larger than that of the second type of capillary wick and the first type of capillary wick is made by stacked metal sheets.

1. FIELD OF THE INVENTION

The present invention relates generally to a heat transfer apparatus, and more particularly to a heat pipe having a composite capillary wick structure.

2. DESCRIPTION OF RELATED ART

As a heat transfer apparatus, heat pipes can transfer heat rapidly and therefore are widely used in various fields for heat dissipation purposes. For example, in the electronics field, heat pipes are commonly used to transfer heat from heat-generating electronic components, such as central processing units (CPUs), to heat dissipating devices, such as heat sinks. A heat pipe in accordance with the related art generally includes a sealed casing made of thermally conductive material with a working fluid contained in the casing. The working fluid is employed to carry heat from one end of the casing, typically called the “evaporating section”, to the other end of the casing, typically called the “condensing section”. Specifically, when the evaporating section of a heat pipe is thermally attached to a heat-generating electronic component, the working fluid receives heat from the electronic component and evaporates. Then, the generated vapor moves towards the condensing section of the heat pipe under the vapor pressure gradient between the two sections. In the condensing section, the vapor is condensed to liquid state by releasing its latent heat to, for example, a heat sink attached to the condensing section. Thus, the heat is removed from the electronic component.

In order to rapidly return the condensed liquid back from the condensing section to the evaporating section to start another cycle of evaporation and condensation, a capillary wick is generally provided in an inner surface of the casing in order to accelerate the return of the liquid. In particular, the liquid is drawn back to the evaporating section by a capillary force developed by the capillary wick. The capillary wick may be a plurality of fine grooves defined along a lengthwise direction of the casing, a fine-mesh wick, or a layer of sintered metal or ceramic powders. However, the capillary force derived from each type of these wicks is generally different, and the flow resistance provided by each type of wick may also be different. The general rule is that the larger an average capillary pore size a wick has, the smaller the capillary force it develops and the lower a flow resistance it provides.

However, the capillary wick is usually uniformly arranged against the inner surface of the casing from its evaporating section to its condensing section. This singular and uniform-type wick generally cannot provide optimal heat transfer for the heat pipe because it cannot simultaneously obtain a large capillary force and a low flow resistance.

In view of the above-mentioned disadvantage of the heat pipe, there is a need for a heat pipe having good heat transfer.

SUMMARY OF THE INVENTION

A heat pipe in accordance with a preferred embodiment of the present invention includes a metal casing and a composite capillary wick arranged on an inner surface of the casing. The casing has an evaporating section, a condensing section and a central section between the evaporating section and the condensing section. A first type of capillary wick is provided in the central section and a second type of capillary wick is provided in the evaporating section. The capillary pore size of the first type of capillary wick is larger than that of the second type of capillary wick.

Other advantages and novel features of the present invention will become more apparent from the following detailed description of preferred embodiment when taken in conjunction with the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present apparatus and method can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present apparatus and method. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.

FIG. 1 is a longitudinal cross-sectional view of a heat pipe in accordance with a first embodiment of the present invention;

FIG. 2 is a radial cross-sectional view of the heat pipe in accordance with the first embodiment, taken along line II-II of FIG. 1;

FIG. 3 is a longitudinal cross-sectional view of a heat pipe in accordance with a second embodiment of the present invention;

FIG. 4 is a longitudinal cross-sectional view of a heat pipe in accordance with a third embodiment of the present invention; and

FIG. 5 is a longitudinal cross-sectional view of a heat pipe in accordance with a fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a heat pipe in accordance with a first embodiment of the present invention. The heat pipe comprises a casing 100 and a composite capillary wick 200 arranged to attach on an inner wall of the casing 100. A column-shaped vapor passage 300 is enclosed with an inner surface of the composite capillary wick 200 in a center of the casing 100. The casing 100 comprises an evaporating section 400 at one end and a condensing section 600 at an opposite end thereof, and a central section (i.e. adiabatic section) 500 located between the evaporating section 400 and the condensing section 600. The casing 100 is made of highly thermally conductive materials such as copper or copper alloys and filled with a working fluid (not shown), which acts as a heat carrier for carrying thermal energy from the evaporating section 400 to the condensing section 600. Heat that needs to be dissipated is first transferred to the evaporating section 400 of the casing 100 to cause the working fluid to evaporate. Then, the heat is carried by the working fluid in the form of vapor to the condensing section 600 where the heat is released to ambient environment, thus condensing the vapor into liquid. The condensed liquid is then brought back via the composite capillary wick to the evaporating section 400 where it is again available for evaporation.

The composite capillary wick 200 comprises a first type of capillary wick 250 which is a folded-type wick and a second type of capillary wick 240 which is one of a sintered-type wick and mesh-type wick. The first type of capillary wick 250 is defined on the inner wall of the casing 100 at the central and condensing sections 500, 600. Referring to FIG. 2, the first type of capillary wick 250 is formed by a plurality of metal sheets 222 tightly stacked together along a radial direction of the casing 100 with an outer metal sheet being attached to the inner wall of the casing 100. Each metal sheet 222 is folded with a cross-sectional configuration having a plurality of serrations disposed along a circle so as to form the first type of capillary wick 250 with a beehive-shaped structure in a radial direction of the casing 100. The metal sheet 222 can be stamped to define a plurality of pores or form a plurality of protruding portions. The capillary pore size and rate of the first type of capillary wick 250 is accurately controlled during manufacturing of the metal sheets 222. The first type of capillary wick 250 has a large pore size to make the condensed liquid return to the evaporating section 400 quickly and simultaneously lowers a flow resistance caused by adverse contact between the vapor and liquid at the central section 500 of the casing 100. The second type of capillary wick 240 is arranged on the inner wall of the casing 100 at the evaporating section 400 and has a high pore rate and a small pore size; as a result the liquid at the evaporating section 400 is then rapidly evaporated and the condensed liquid is rapidly drawn back to the evaporating section 400. The capillary pore size of the first type of capillary wick 250 is larger than that of the second type of capillary wick 240 so that the first type of capillary wick 250 at the central section 500 can provide a smaller flow resistance than the second type of capillary wick 240 at the evaporating section 400 for the condensed liquid as the condensed liquid is brought back to the evaporating section 400 via the central section 500.

In this embodiment, the composite capillary wick 200 has different types of capillary wick disposed in different sections of the heat pipe. The first type of capillary wick 250 has a relatively large average capillary pore size and therefore provides a relatively low flow resistance to the condensed liquid to flow therethrough, and meanwhile, the second type of capillary wick 240 has a relatively small average capillary pore size and accordingly develops a relatively large capillary force to the liquid. As a result, the first type of capillary wick 250 reduces the flow resistance that the condensed liquid encounters when flowing through the condensing and central sections 600, 500, and the second type of capillary wick 240 has a large capillary force and therefore the liquid is then rapidly drawn back to the evaporating section 400 from the central section 500 as the liquid reaches to a position adjacent to the evaporating section 400. The condensed liquid is returned back from the condensing section 600 in an accelerated manner. After the condensed liquid is returned back to the evaporating section 400, a next phase-change cycling will then begin. Thus, as a whole, the cycling of the working fluid is accelerated and therefore the total heat transfer capacity of the heat pipe is enhanced.

FIG. 3 illustrates a heat pipe in accordance with a second embodiment of the present invention. Main differences between the first and second embodiments are that in the second embodiment the composite capillary wick 210 comprises a second capillary wick 241 similar to the second type of capillary wick 240 in the first embodiment, a first type of capillary wick 251 similar to the first type of capillary wick 250 in the first embodiment and a third capillary wick 261. The second capillary wick 241 is arranged at the evaporating section 400 of the casing 100. The first type of capillary wick 251 is located at the central section 500 of the casing 100. The third capillary wick 261 is disposed at the condensing section 600 of the casing 100. The third capillary wick 261 is a sintered-type wick and has a larger capillary pore size than that of the second type of capillary wick 241 in the second embodiment so as to provide a high pore size and a low flow resistance for the condensing section 600 of the casing 100.

FIG. 4 illustrates a heat pipe in accordance with a third embodiment of the present invention. Main differences between the third and first embodiments are that in the third embodiment the thickness of the composite capillary wick 220 from the condensing section 600 to the evaporating section 400 via the central section 500 is gradually increased. The thickest point of the composite capillary wick 220 is received at the evaporating section 400 of the casing 100 so as to provide a large capillary wick force and absorb more of the working fluid at the evaporating section 400. The vapor passage 310 enclosed by the composite capillary wick 220 has a gradually decreasing radial cross-sectional area along a longitudinal direction of the casing 100 from the condensing section 600 toward the evaporating section 400. The other structure of the heat pipe of the third embodiment is similar to that of the first embodiment. Heat exchange between the working fluid and the inner wall of the casing 100 is greatly improved and the heat transfer efficiency of the heat pipe is improved as a result.

FIG. 5 illustrates a heat pipe in accordance with a fourth embodiment of the present invention. Main differences between the fourth and second embodiments are that in the fourth embodiment the thickness of the composite capillary wick 230 from the condensing section 600 to the evaporating section 400 via the central section 500 is gradually increased. The thickest point of the composite capillary wick 230 is received in the evaporating section 400 of the casing 100 so as to provide a large capillary wick force and absorb more of the working fluid at the evaporating section 400. The vapor passage 320 enclosed by the composite capillary wick 230 has a gradually decreasing radial cross-sectional area along a longitudinal direction of the casing 100 from the condensing section 600 toward the evaporating section 400. The other structure of the heat pipe of the third embodiment is similar to that of the second embodiment.

It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. 

1. A heat pipe comprising: a metal casing containing a working fluid therein, the casing comprising an evaporating section at one end and a condensing section at an opposite end thereof, and a central section located between the evaporating section and the condensing section; a first type of capillary wick arranged in an inner wall of the casing at the central section, the first type of capillary wick being formed by metal sheets; and a second type of capillary wick disposed on the inner wall of the casing at the evaporating section; wherein a capillary pore size of the first type of capillary wick is larger than that of the second type of capillary wick and a vapor passage is enclosed by the first and second types of the capillary wick.
 2. The heat pipe of claim 1, wherein the metal sheets are tightly stacked together along a radial direction of the casing with an outer metal sheet being attached to the inner wall of the casing.
 3. The heat pipe of claim 2, wherein each metal sheet is folded and the first type of capillary wick has a beehive-shaped structure in the radial direction of the casing.
 4. The heat pipe of claim 3, wherein the first type of capillary wick is also arranged in the condensing section of the casing.
 5. The heat pipe of claim 4, wherein thicknesses of the first and second types of capillary wick from the condensing section to the evaporating section via the central section are gradually increased and the vapor passage has a gradually decreasing radial cross-sectional area along a longitudinal direction of the casing from the condensing section toward the evaporating section.
 6. The heat pipe of claim 4, wherein the second type of capillary wick is one of sintered-type wick and mesh-type wick.
 7. The heat pipe of claim 3, wherein a sintered-type wick is disposed in the condensing section of the casing and has a capillary pore size larger than that of the second type of capillary wick at the evaporating section of the casing.
 8. The heat pipe of claim 7, wherein thicknesses of the capillary wicks from the condensing section to the evaporating section via the central section are gradually increased and the vapor passage has a gradually decreasing radial cross-sectional area along a longitudinal direction of the casing from the condensing section toward the evaporating section.
 9. The heat pipe of claim 8, wherein the second type of capillary wick has a thickest point at the evaporating section of the casing in a radial direction of the casing.
 10. The heat pipe of claim 3, wherein each metal sheet has a cross-sectional configuration with a plurality of serrations disposed along a circle. 